Formation of silicon nitride film for a phase shift mask at 193 nm

ABSTRACT

A half tone phase shift mask material, suitable for use at 193 nm is disclosed. It comprises a layer of nitrogen rich silicon nitride that was formed by subjecting a mixture of a nitrogen bearing gas, such as nitrogen and/or ammonia, with a silicon bearing gas, such as silane, to a plasma discharge. Provided the ratio of the nitrogen bearing to the silicon bearing gases is about 10 to 1, films having the required optical properties at 193 nm are formed. These properties are a reflectance that is less than 15% and a transmittance that is between 4 and 15%. Related optical properties, namely an extinction coefficient of about 0.4 and a refractive index of about 2.5, are also closely approached. Additionally, the films are stable under prolonged UV exposure and exhibit good etch behavior.

This is a division of patent application Ser. No. 09/097,145, filingdate Jun. 12, 1998, now U.S. Pat. No. 6,045.954 Formation Of SiliconNitride Film For A Phase Shift Mask At 193 Nm., assigned to the sameassignee as the present invention.

FIELD OF THE INVENTION

The invention relates to phase shift mask material suitable for use at193 nm and to methods for manufacturing same.

BACKGROUND OF THE INVENTION

As the dimensions within integrated circuits continue to shrink, theresolution limit of the radiation used to expose photoresist continuesto be the final barrier to shrinking dimensions even further. Theintroduction of the phase shift mask has allowed this barrier to bepushed towards smaller and smaller features. The principle on which aphase shift mask operates is to bring about the destructive interferenceof the diffraction fringes normally found at the edge of an image,thereby improving image contrast.

For lithography at 365 nm (i-line) and 248 nm (for example from a KrFlaser) the attenuated phase shifting mask (APSM) has been widely usedbecause of its ease of design and fabrication. A wide range of materialssuitable for use as APSMs are available, including silicon oxide,chromium oxide, silicon nitride, silicon carbo nitride, and molybdenumsilicon oxynitride. Application is usually focussed on contact layersdue to alleviation of defect problems in dark field masks. It can pushthe resolution of contact printing one generation ahead since aerialimage contrast is enhanced.

When, however, the wavelength of the resist exposure radiation isreduced still further to 193 nm (ArF laser), the materials listed aboveare no longer adequate and at least one replacement must be found ifefficient phase shift masks for use at 193 nm are to be developed. Thisis because the refractive index (n) and the extinction coefficient areboth functions of wavelength.

The two key optical properties that must be possessed by a material ifit is to be suitable for use as a phase shift mask at 193 nm are thatits transmittance (at a thickness corresponding to a 180 degree phaseshift) must be between about 4 and 15% while its reflectance must beabout 15%. These properties, in turn, depend on the refractive index andextinction coefficient of the material in question (at 193 nm).

Referring now to FIG. 1, we show three curves, measured at 193 nm, ofextinction coefficient (attenuation of transmitted radiation due to bothabsorption and scattering) as a function of refractive index. Curve 11is for a transmittance of 4% (corresponding to a relatively thick film),curve 12 is for a transmittance of 15% (corresponding to a relativelythin film), and curve 13 is for a reflectance of 15% (independent ofthickness).

Thus for a material to be suitable for use as a phase shift mask at 193nm its optical properties must be such that it is located within thearea marked as 15 in FIG. 1, as close to line 13 as possible. Thus, oneset of optimal properties would be an extinction coefficient of about0.4 and a refractive index of about 2.5.

The point marked as SiN_(x) in FIG. 1 indicates where non-stoichiometricsilicon nitride (having the desired properties) falls on this plot. Itis clear from this that non-stoichiometric silicon nitride is what isneeded for a 193 nm APSM. This conclusion is confirmed by the data shownin FIG. 2 which is for stoichiometric silicon nitride. As can be seen,at 193 nm, the refractive index is 2.65 (curve 21) while the extinctioncoefficient is about 0.17.

We refer now to FIG. 3 which is similar to FIG. 2 in that curves ofrefractive index and extinction coefficient, as a function ofwavelength, for silicon nitride are also plotted. These samples weredeposited by means of sputtering and have been reported by B. W. Smithet al. in SPIE 1997, vol. 3051, pp. 236-244. In the course oftransferring silicon nitride from a target to a substrate, some of thenitrogen was lost so the films ended up as silicon rich. The effects ofthis departure from stoichiometry are reflected in the values forrefractive index and extinction coefficient at 193 nm. These were about2.37 and 0.38, respectively, much closer to the ideal values of 2.5 and0.4 than could be achieved with non-stoichiometric films.

While these optical values for silicon rich films are attractive, otherproperties of the silicon rich films, such as UV stability and etchcharacteristics, make them unattractive for use within an integratedcircuit manufacturing process. Other attempts to modify the opticalproperties of silicon nitride by varying the silicon/nitrogen ratio havealso been reported but it has turned out that silicon rich siliconnitride is much easier to form than a nitrogen rich version. Forexample, Hess et al. (U.S. Pat. No. 4,863,755 September 1989) show howPECVD may be used to generate silicon nitride films. They use an organicsilicon/nitrogen source in conjunction with hydrogen or ammonia. Somecontrol of stoichiometry is achieved by varying process conditions butthe films also include significant quantities of carbon and oxygen.

Hubler et al. (U.S. Pat. No. 5,015,353 May 1991) teach how films ofcomposition Si_(1−x)N_(x), where x is controlled to be between 0 and0.57, can be formed. Thus any departure from stoichiometry that theyachieved was always on the side of being silicon rich.

Dowben (U.S. Pat. No. 5,468,978 November 1995) also teaches that thestoichiometry of films produced by PECVD may be controlled by varyingconcentration of precursor materials and other process parameters.However the material produced was boron carbide so a direct comparisonwith silicon nitride is not possible.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a half tonephase shift mask material suitable for use at 193 nm.

Another object of the invention has been to provide a process formanufacturing said material.

A further object of the invention has been to provide a process forforming a silicon nitride film that is nitrogen rich.

These objects have been achieved by subjecting a mixture of a nitrogenbearing gas, such as nitrogen and/or ammonia, with a silicon bearinggas, such as silane, to a plasma discharge thereby causing thedeposition of silicon nitride films. Provided the ratio of the nitrogenbearing to the silicon bearing gases is about 10 to 1, films having therequired optical properties at 193 nm are formed. These properties are areflectance that is less than 15% and a transmittance that is between 4and 15%. Related optical properties, namely an extinction coefficient ofabout 0.4 and a refractive index of about 2.5, are also closelyapproached. Additionally, the films are stable under prolonged UVexposure and have good etch behavior.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of extinction coefficient vs. refractive index forcertain values of reflectance and transmittance.

FIG. 2 is a plot of refractive index and extinction coefficient as afunction of wavelength, for a layer of stoichiometric silicon nitride at193 nm.

FIG. 3 is similar to FIG. 2 but is for silicon nitride that isunder-stoichiometric.

FIGS. 4 and 5 plot refractive index and extinction coefficient as afunction of the ratio between the nitrogen bearing and silicon bearinggases used in the PECVD.

FIG. 6 plots reflectance and transmittance as functions of the ammoniato silane ratio during PECVD.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses how nitrogen rich films of siliconnitride may be formed. The method used for depositing the films isPlasma Enhanced Chemical Vapor Deposition (PECVD). As discussed above,the use of conventional PECVD or other known deposition techniques leadsto films that are silicon rich. In the present invention it is shownthat, if key process parameters are confined to certain critical ranges,good quality nitrogen rich silicon nitride can be obtained.

The process of the present invention begins with the admission ofgaseous compounds containing nitrogen and silicon into a reactionchamber. Typically, the nitrogen component has been a mixture ofnitrogen and ammonia while the silicon component has been silane, but itwill be understood that other nitrogen and silicon bearing gases couldbe substituted without departing from the spirit of the invention.

Following standard practice, the pressure of the gases in the chamber iscontrolled by establishing a dynamic equilibrium between a pumpingsystem that continuously removes gases from the chamber, and flow valvesthat continuously admit gases into it. Once the pressure has beenstabilised (at between about 10⁻³ and 10⁻¹ torr) a plasma discharge (ata power level between about 90 and 120 watts) is set up in the gas. Thisenhances the CVD process whereby a layer of silicon nitride deposits ona suitable substrate within the chamber. Typically, the substrate wouldbe a silicon wafer which bears a partially completed integrated circuit.Substrate temperature is generally between about 250 and 300° C.

Deposition of the silicon nitride film is allowed to continue until itreaches a thickness that would cause a 180 degree phase shift in a beamof 193 nm radiation passing through it. This thickness is between about720 and 750 Angstroms and, once it has been reached, the process isterminated.

Key to the ability of the above described process to yield nitrogen richsilicon nitride films having the required optical and other propertiesis the control of certain key parameters, notably the relative flowrates of the nitrogen, ammonia and silane gases:

For a given mix of ammonia and nitrogen, the flow rate of the mixrelative to the flow rate of the silane should be between about 10 and12 to 1. Within the mix, the flow rate of the nitrogen relative to theammonia should be between about 0.9 and 1.1 to 1. This is equivalent toa nitrogen:ammonia:silane flow rate ratio of between about 4.5 to6.6:4.5 to 6.6:0.9 to 1.1.

In more limited terms, relating to specific flow rates, if the nitrogenis admitted at a flow rate between about 45 and 70 SCCM, the ammoniamust be admitted at a flow rate between about 40 and 70 SCCM, and thesilane must be admitted at a flow rate between about 9 and 11 SCCM.

If the above process guide-lines are strictly observed, the resultingfilms will be nitrogen rich in that the atomic concentration of nitrogenwill exceed the stoichiometric value of 0.57 by between about 57 and60%.

In FIG. 4 we illustrate the influence of the ammonia/nitrogen to silaneratio on the refractive index and extinction coefficient of the films.Curve 42 is for the extinction coefficient and a ratio of about 10:1 isseen to be optimum for achieving the ideal of 0.4 as already noted.Curve 41 is for the refractive index. At the 10:1 ratio t his curve isseen to be almost flat and corresponding to a value of about 2.3. Thenitrogen fraction was 0.04 and the measurements were made at 193 nm.

In FIG. 5 we display the results of a process using pure ammonia andsilane. The influence of the ammonia to silane ratio on the refractiveindex and extinction coefficient of the films has been measured at 193nm. Curve 52 is for the extinction coefficient and a ratio of about 12:1is seen to be optimum for achieving the ideal of 0.4 as already noted.Curve 51 is for the refractive index. At the 12:1 ratio its value isabout 2.32.

In FIG. 6 the same data seen in FIG. 5 is shown in terms of reflectanceand transmittance, also measured at 193 nm. Curve 61 is for thetransmittance and a ratio of about 12:1 is seen to correspond to atransmittance of about 10% (which is between 4 and 15%). Curve 62 is forthe reflectance and a ratio of about 12:1 is seen to correspond to avalue of about 10%, (i.e. less than 15%.

Thus, silicon nitride films deposited according to the aboveguide-lines, namely an ammonia/nitrogen to silane ratio between about 10and 12 to 1 or an ammonia to silane ratio between about 9 and 12 to 1,used as part of a PECVD process, will lead to silicon nitride films thatare nitrogen rich and have transmittances between 4 and 15% and areflectance that is less than 15%. Additionally, such films have beenfound to be stable under prolonged UV exposure and to exhibit good etchbehaviour (providing an etch selectivity in excess of 6).

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A process for forming a nitrogen rich layer ofsilicon nitride, comprising: admitting nitrogen, ammonia, and silanegases into a reaction chamber, at a pressure and in fixed relativeconcentrations; and initiating a plasma discharge within said gasesthereby causing a layer of nitrogen rich silicon nitride to form,through plasma enhanced chemical vapor deposition, on a substrate. 2.The process of claim 1 wherein the step of admitting the gases in fixedrelative concentrations further comprises adjusting and maintaining aflow rate for each gas.
 3. The process of claim 2 wherein relative flowrates for nitrogen, ammonia, and silane of between about 4.5 and 6.6 tobetween about 4.5 and 6.6 to between about 0.9 and 1.1, respectively aremaintained.
 4. The process of claim 1 wherein the pressure is betweenabout 10⁻³ and 10⁻¹ torr.
 5. The process of claim 1 wherein the plasmadischarge is at a power level between about 90 and 120 watts.
 6. Theprocess of claim 1 wherein the substrate is at a temperature betweenabout 250 and 300° C.
 7. The process of claim 1 wherein the layer ofnitrogen rich silicon nitride has a refractive index at 193 nm that isbetween about 2.29 and 2.31.
 8. The process of claim 1 wherein the layerof nitrogen rich silicon nitride has an extinction coefficient at 193 nmthat is between about 0.36 and 0.53.
 9. The process of claim 1 whereinthe nitrogen is admitted at a flow rate between about 45 and 66 SCCM,the ammonia is admitted at a flow rate between about 45 and 66 SCCM, andthe silane is admitted at a flow rate between about 9 and 11 SCCM.